1. Field of the Invention
The present invention relates to a tool joint and a drill pipe made therefrom. More particularly, the present invention relates to a composite tool joint comprised of dissimilar metals having different moduli of elasticity.
2. Description of the Prior Art
As the search for gaseous and/or liquid hydrocarbons expands, new drilling techniques are being used, wells are being drilled to greater and greater depths, and such hydrocarbons are being recovered from subterranean formations that are extremely hostile in the sense of containing highly corrosive gases and liquids. For example, in the recent past, there has been a dramatic increase in the amount of directional drilling including the drilling of so-called horizontal wells in which a well borehole is first drilled to a desired vertical depth and the well is then "kicked off" in a direction transverse to the vertical borehole and, in many cases, at right angles thereto. In many instances in these horizontal wells, the radius of curvature is quite short. For example, it is not uncommon for the drill pipe to be drilling in holes that change direction at the rate of approximately 30.degree. per 100 feet.
As is well known, a drill pipe generally comprises threaded pipe connections known as "tool joints" that are interconnected by an elongate central portion, commonly referred to as the "pipe body." Typically, the tool joint connected to one end of the pipe body has a box connection while the tool joint connected to the other end of the pipe body has a pin connection. This allows successive lengths of drill pipe to be adjoined to one another to form the drill string.
As disclosed in the prior art, there are numerous methods of interconnecting the tool joints to the pipe body to form a length of drill pipe. One method of accomplishing this, and as disclosed in U.S. Pat. No. 4,445,265, incorporated herein by reference for all purposes, is what is known as a threaded shrink-grip connection. In this method, the ends of the pipe body are threaded, forming pins. Tool joints on each end of the pipe body are provided with a threaded box into which the pins are threadedly received. Typically, the box is slightly undersized so that the connection between the pin on the pipe body and box in the tool joint cannot be made until the tool joint is heated sufficiently to cause a desired amount of thermal expansion. The tool joint, thus heated, is then threaded onto the pin of the pipe body and cooled so that, as the former cools, it shrinks onto the pipe body, forming a tight, rigid, sealed connection.
As noted, many horizontal wells involve short radius curves. While the radius of bend in the pipe body of the drill pipe will generally follow the radius of curvature of the borehole, the sectional properties of the tool joint inherently make it stiff and prevent it from any significant bending in these curved boreholes. Bending of the pipe body occurs especially where it is adjoined to the tool joint, which leads to fatigue failures in this area. The bending stress in pipe is directly proportional to the modulus of elasticity of the pipe and inversely proportional to the radius of curvature of the borehole. Accordingly, for a drill pipe that is made from a material having a high modulus of elasticity, bending stresses become very high in a short-radiused borehole. Thus, drill pipe wherein the pipe body and the tool joints are made of steel exhibit high bending stresses in short-radiused boreholes since steel has a modulus of elasticity of approximately 30 million psi. On the other hand, titanium has a modulus of elasticity of 17 million psi. Accordingly, titanium is much more flexible than steel and is subject to lower bending stress than steel under the same conditions. Furthermore, titanium is lighter weight than steel, which decreases frictional losses when the drill string is being pushed or pulled through non-vertical holes. As is also well known, titanium is also more corrosion-resistant than steel and accordingly is much preferred in cases where the well contains corrosion-inducing fluids. Thus, while drill pipe made of titanium pipe bodies and steel tool joints would be much preferred in directional drilling for reasons mentioned above, it has been found that prior art titanium drill pipe employing a titanium pipe body connected to a steel tool joint by a threaded shrink-grip connection exhibited low fatigue life generally at the juncture of the titanium pipe body and the steel tool joint.